Raptor_Guy

I just had this problem. I see that the modpack is 1.12.2 like mine is. What I didn't realize is that forge version 2854 (the latest and recommeded version for 1.12.2), you have to download the installer file, not the universal file as you have had to do with previous versions. Don't run it or anything, and still rename it to modpack.jar
Hope this helps.

Forge changed things with 2854 and newer, you must use the Installer now, not the universal. You still need to re-name it to Modpack. And don't run the installer, just re-name it.